P-dichlorobenzene is an important raw material for production of polyphenylene sulfide. Various processes for production of p-dichlorobenzene have been developed, however, a simpler process that results in lower costs and high yields is still needed. Conventional commercial processes for chlorinating benzene or monochlorobenzene to dichlorobenzene produce about 3/2 para/ortho (P/O) isomer ratio, and so the efficiency of the processes suffer from inherent limitations because of the large amount of unwanted o-dichlorobenzene produced. An improved process with greater efficiency in which the para/ortho (P/O) isomer ratio is 9/1 or greater is of great utility.
Konishi et al, in Regioselective Para-Chlorination of Alkylbenzenes on Chemically Modified Silica Surfaces, Chemistry Letters, 1980, pp. 1423-1426, describe the chlorination of alkylbenzenes with chlorine in carbon tetrachloride in the presence of chemically modified silica catalysts. The silica catalysts showed higher selectivity than FeCl.sub.3.
Peter Kovacic et al, Chlorination of Aromatic Compounds with Metal Chlorides, Journal of the American Chemical Society, Vol. 76, Nov. 5, 1954, pp. 5491-5494, teach the use of metal chlorides to halogenate aromatic compounds. Chlorination of chlorobenzene with chlorine gas using ferric chloride as a catalyst gave a P/O ratio of about 54 percent. FeCl.sub.3 alone gave a ratio of about 88 percent.
Herbert F. Wiegandt et al, in Improved Yields of p-Dichlorobenzene, Industrial and Engineering Chemistry, September 1951, pp. 2167-2172, teach the batch and continuous production of dichlorinated benzenes. Effective catalysts were found to be AlCl.sub.3, FeCl.sub.3, I.sub.2, SbCl.sub.5, Fe powder, SnCl.sub.4 ; optimum reaction temperatures were determined. Batch chlorination in the liquid phase and continuous chlorination with a uniform suspension of hydrocarbon/catalyst is discussed.
Walter Prahl et al, in U.S. Pat. No. 1,963,761, teaches a process of making chlorobenzene from benzene where dichlorobenzene is also produced; vaporized benzene, hydrogen chloride and oxygen with or without a carrier gas are passed over a contact substance of copper, metals or their compounds (e.g. FeCl.sub.3, CuCl).
G. A. Webb, in U.S. Pat. No. 2,527,606, teaches the production of p-dichlorobenzene from benzene. The process uses catalysts such as aluminum chloride, zinc chloride, iron chloride, or metallic fluorides. Approximately equal parts of benzene and monochlorobenzene are added. The benzenes and catalyst flow countercurrent to chlorine in a reactor.
W. A. White et al, in U.S. Pat. No. 3,029,296, conceived of a process in which FeCl.sub.3 was used as a reagent but the spent FeCl.sub.3 (or FeCl.sub.2, ferrous chloride) was moved from the reactor to a chlorinator where it was treated with chlorine to regenerate FeCl.sub.3, which was then transported back to the reactor for use again as a chlorinating reagent. This concept provides P/P+O ratios of 92 to 96 percent. However, high costs are involved in solid materials handling. Further other problems involved the transporting of FeCl.sub.3 /FeCl.sub.2 back and forth between the reactor and chlorinator that resulted in "fines" which tend to cause blockage in the reactor because of the differences in molecular volume (or density) that results from cycling back and forth between FeCl.sub.3 and FeCl.sub.2.
Adolf Wissner et al, in U.S. Pat. No. 4,300,004, teach the separation of ortho-, meta-. and para-dichlorobenzene from an isomeric mixture thereof.
Japanese patent Kokai 74/76,828 teaches the batch and continuous production of monochlorotoluene. The reaction is in the vapor phase. In the batch system, vaporized toluene contacts with ferric chloride to produce chlorotoluene. When all FeCl.sub.3 is consumed, chlorine gas is introduced to regenerate the ferric chloride. When ferric chloride is regenerated toluene is again introduced. In the continuous operation. chlorine gas and toluene are simultaneously reacted over ferric chloride. The process is stated to be applicable to chlorobenzene.